1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device including a bonding pad and, more particularly, to a method of manufacturing a semiconductor device including a bonding pad having a main surface on which a passive state film is formed and the semiconductor device.
2. Description of the Background Art
A semiconductor device includes a number of semiconductor elements formed on a semiconductor substrate. Each of the semiconductor elements is electrically connected through an interconnection layer to a bonding pad. Wire bonding is one of the methods of electrically connecting the bonding pad and external wiring.
FIG. 23 is a typical diagram of a semiconductor device in which wire bonding is performed. Bonding pads 11 are formed on a main surface of a silicon substrate 1. Each of bonding pads 11 is electrically connected through a bonding wire 7 to an external lead 5. Silicon substrate 1 is fixed on a die pad 3, Silicon substrate 1 is sealed with a resin 9.
FIG. 24 is an enlarged cross sectional view of a part A in FIG. 23. A DRAM (Dynamic Random Access Memory) element 13 is formed on the main surface of silicon substrate 1. A first insulating film 15 is formed to cover DRAM element 13. First interconnection films 17 and bonding pad 11 are formed on first insulating film 15. First interconnection films 17 and bonding pad 11 are formed at the same time by patterning a conductive film formed on first insulating film 15. One of first interconnection films 17 is electrically connected, through a contact hole 29 formed in first insulating film 15, to DRAM element 13.
Each of first interconnection films 17 is of a layered structure including an aluminum alloy film 23 and a barrier metal film 25. Aluminum alloy film 23 is formed of aluminum with silicon, copper, or the like added thereto. Barrier metal film 25 is formed of titanium nitride (TIN), titanium tungsten (TiW), or the like. Barrier metal film 25 underlies aluminum alloy film 23 and prevents the aluminum from invading silicon substrate 1 due to sinter so as to prevent an alloy spike phenomenon.
A surface protecting film 19 is formed on first interconnection films 17 and bonding pad 11. A buffer coat film 21 is formed on surface protecting film 19. Buffer coat film 21 is formed for the purpose of protecting the semiconductor element from the stress of resin 9. An opening 27 is formed on main surface 12 of bonding pad 11, and bonding wire 7 is bonded to main surface 12.
A method of manufacturing the semiconductor device illustrated in FIG. 24 will be described in the following. As illustrated in FIG. 25, an element isolating insulating film 43 is formed on a silicon substrate 1. Then, a DRAM element 13 is formed on an exposed part of silicon substrate 1. DRAM element 13 includes a transfer gate electrode 31, an impurity diffused layer 33, a word line 35, a storage node 37, a capacitor insulating film 39, and a cell plate 41.
As illustrated in FIG. 26, a first insulating film 15 is formed on the whole surface of silicon substrate 1. Then, a contact hole 29 is formed on impurity diffused layer 33 using photolithography technique and etching technique.
Referring to FIG. 27, a barrier metal film 25 and an aluminum alloy film 23 are formed sequentially on first insulating film 15 by sputtering. Then, barrier metal film 25 and aluminum alloy film 23 are patterned using photolithography technique and etching technique to form a bonding pad 11 and first interconnection films 17. Then, a surface protecting film 19 is formed to cover bonding pad 11 and first interconnection films 17. Then, surface protecting film 19 on a main surface 12 of bonding pad 11 is removed using photolithography technique and etching technique to form an opening 45.
As illustrated in FIG. 28, a buffer coat film 21 is formed on surface protecting film 19. Buffer coat film 21 on bonding pad 11 is removed using photolithography technique and etching technique to form an opening 27. Then, silicon substrate 1 is ground from a rear surface of silicon substrate 1 until silicon substrate 1 comes to have a thickness in the range of about 200 .mu.m to about 400 .mu.m. Silicon substrate 1 is cleaned with pure water or the like during the step of grinding silicon substrate 1 and after completion of that step.
As illustrated in FIG. 29, main surface 12 of bonding pad 11 is connected through bonding wire 7 to an external lead (see FIG. 23) by a wire bonding method. Finally, as illustrated in FIG. 30, silicon substrate 1 is sealed with a resin 9.
A semiconductor device completed as described above is actually used; however, since resin 9 does not have the property of completely inhibiting moisture from passing therethrough, it happens that moisture in an external atmosphere invades resin 9. As illustrated in FIG. 31, resin 9 is directly in contact with bonding pad 11 in a part 49. Moisture invading resin 9 corrodes bonding pad 11 from part 49 to cause a corrosion hole 47 to be generated in bonding pad 11. If corrosion hole 47 grows, there is the disadvantage of breaking off the connection between bonding pad 11 and bonding wire 7.
Japanese Patent Laying-Open No. 63-269541 (1988) discloses a semiconductor device in which a passive state film is formed on a bonding pad, which will be described in the following. FIG. 32 is a partial perspective view of a semiconductor device disclosed in Japanese Patent Laying-Open No. 63-269541 (1988). An insulating protective film 59 and a bonding pad 55 of aluminum are formed on a semiconductor substrate 53. Semiconductor substrate 53 in this state is immersed in warm water to form a passive state film (Al.sub.2 O.sub.3) 57 on bonding pad 55. Then, a bonding wire 51 is bonded through passive state film 57 to bonding pad 55. Since passive state film 57 has the property of not letting moisture therein, it does not happen that moisture externally invading the resin corrodes bonding pad 55.
When a large current stress is applied to a metal, metal atoms move. This phenomenon is referred to as electromigration. If electromigration is generated, local voids are generated in interconnection, so that the resistance of the interconnection is increased, or disconnection occurs. In order to prevent electromigration, an aluminum alloy, which is obtained by adding a metal such as copper or the like to aluminum, is used to form an interconnection layer. The interconnection layer and the bonding pad are formed by patterning a conductive film formed on an insulating film, so that a metal such as copper or the like is also added to the bonding pad.
If a passive state film 67 is formed on a bonding pad formed of an aluminum alloy including copper in accordance with the method of Japanese Patent Laying-Open No. 63-269541, a number of pinholes 69 are generated in passive state film 67 as illustrated in FIG. 33. The reason why pinholes 69 are generated will be described in the following. In FIG. 33, reference numeral 61 denotes a bonding pad, reference numeral 63 denotes aluminum, and reference numeral 65 denotes copper.
FIG. 34 is an enlarged cross sectional view of a part of FIG. 33. Copper is added to aluminum, and a part of the added copper is exposed on the surface of the aluminum. If bonding pad 61 in this state is immersed in warm water to form a passive state film (Al.sub.2 O.sub.3) 67, so-called local battery action occurs. Specifically, the copper forms a plus Dole, the aluminum forms a minus pole, current flows between the copper and the aluminum, and a part of the aluminum around the copper fuses. Since the part of the aluminum around the copper fuses, passive state film 67 is not formed on the part of the aluminum around the copper, and pinhole 69 is generated. If a number of pinholes are formed in passive state film 67, moisture invading the resin comes into contact with the aluminum in the pinholes and corrodes the aluminum. In addition, while the passive state film preferably has a thickness of 50-200 .ANG., the local battery action prevented the passive state film from having a thickness of approximately 50-200 .ANG. on the bonding pad with added copper.
The above-described Japanese Patent Laying-Open No. 63-269541 (1988) discloses another method of forming a passive state film, in which a bonding pad is immersed in a hydrogen peroxide liquid. According to this method, a passive state film (Al.sub.2 O.sub.3) is formed on a bonding pad by a reaction as follows: EQU 2Al+3H.sub.2 O.sub.2 .fwdarw.Al.sub.26l O.sub.3 +3H.sub.2 O
In a case where a hydrogen peroxide liquid is used, the speed of formation of a massive state film is higher than the speed of fusion of aluminum due to the local battery action, so that it is possible to form a passive state film with no pinholes.
However, in a case where a hydrogen peroxide liquid is used, as the reaction proceeds, the hydrogen peroxide concentration becomes lower, and the oxidation power becomes weaker. Therefore, in a part where the passive state film is not formed immediately, the speed of fusion of the aluminum becomes higher than the speed of formation of the passive state film, so that pinholes are generated.